1. Field of the Invention
This invention relates generally to the design and manufacture of plastic containers, particularly plastic containers that are made by the reheat stretch blow molding process.
2. Description of the Related Technology
Plastic containers for packaging beverages are commonly fabricated from polyesters such as polyethylene terephthalate (PET). PET containers are lightweight, inexpensive, and recyclable and can be economically manufactured in large quantities.
PET containers are typically manufactured using the stretch blow molding process. This involves the use of a preform that is injection molded into a shape that facilitates distribution of the plastic material within the preform into the desired final shape of the container. The preform is first heated and then is longitudinally stretched and subsequently inflated within a mold cavity so that it assumes the desired final shape of the container. As the preform is inflated, it takes on the shape of the mold cavity. The polymer solidifies after contacting the cooler surface of the mold, and the finished hollow container is subsequently ejected from the mold.
PET containers are common for use in packaging beverages such as juices using what is known in the industry as the hot-fill process. This involves filling the containers while the liquid product is at an elevated temperature, typically 68° C.-96° C. (155° F.-205° F.) and usually about 85° C. (185° F.) in order to sterilize the container at the time of filling. Containers that are designed to withstand the process are known as “hot fill” type containers. After filling, such containers undergo significant volumetric shrinkage as a result of the cooling of the product within the sealed container. Hot fill type containers accordingly must be designed to have the capability of accommodating such shrinkage. Typically this has been done by incorporating one or more concave vacuum panels into the side wall of the container that are designed to flex inwardly as the volume of the product within the container decreases as a result of cooling. More recently, it has been proposed to accommodate such volumetric shrinkage by providing a movable vacuum panel in the bottom of the container.
In some instances, it is desirable for a plastic container to be formed with a deep inset base, i.e. a base that is shaped to have a relatively tall and narrow standing ring. A deep inset base may be desirable for any one of a number of different reasons, including but not limited to the placement of a movable vacuum panel in the bottom of the container. For example, a manufacturer may desire to place an article in the space that is defined by the container bottom, or a deep inset base may be desirable in order to provide stackability of the containers with respect to each other.
Unfortunately, it has been problematic in the past to manufacture a container having a deep inset base using the reheat stretch blowmolding process. Efforts to produce such containers often resulted in unwanted extreme stretching and thinning of the container wall in the area of the standing ring of the container base, crimping or folding of the standing ring, or other unwanted deformities in the bottom of the container. These problems made it practically impossible to effectively for a container having a deep inset base. A need exists for an improved container having a deep inset base and an improved method for manufacturing such a container.